18 - Visual Processing of Objects II Flashcards
What is fMR adaptation?
An imaging paradigm
Gets around some of the limitations in spatial resolution of the fMRI
BOLD response decreases when stimulus is repeated.
Approximates neural adaptation; repeated firing tires the neurons
If you adapt, then change some aspect of the stimulus (size) and you see recovery from adaptation, a new population of neurons is responding to this new attribute.
If there is no recovery, it implies the same neural population responds irrespective of changes to orientation, size etc.
What is activity in the LOC invariant to?
Size
Position in VF
Image format (grayscale pic vs line drawing, shape defined by motion, luminance, stereo cues)
Visual or tactile input
What type of activity is the LOC sensitive to?
Viewpoint and illumination
Things that can alter the 2D shape that’s projected to the retina
What type of object representation is the LOC used for?
High-level object shape representation
Just underlying shape, not surface cues
What is the LO sensitive to?
Changes in location and size (more than vOT)
2D Shape features
Codes geometry
Sub-region LOtv - visual and haptic input
What is the visual tactile area called and what is it activated and not activated by?
Lateral Occipital Tactile Visual Area LOtv
Activated by seen and palpated objects
not by characteristic sounds of objects
What is the VOT sensitive to?
More invariant to changes in size and location than LO
Sensitive to perceived 3D shape
Correlates with recognition performance
Not activated by haptic input
What does the VOT code for?
Abstract identity representation and mediates awarenses
What is visual agnosia?
a failure to make sense of visual information, to know what it represents.
What is the difference between apperceptive and associate agnosias?
Apperceptive Agnosias
- Recognition deficits linked to problems in perceptual processing
- Cannot differentiate objcts
Associative Agnosias
- Patient can derive normal visual representations but cannot link them to information stored in memory
- Can differentiate things but not say what it is
Describe visual form agnosia
Very severe form, caused by widespread bilateral damage to the occipital lobes (usually from CO poisoning)
Apperceptive Agnosia
Cannot discriminate even simple shapes
Cannot copy drawings
Cannot read
Cannot recognise faces
Typically have a “vision for action” (perception-action dissociation card task)
Describe a perceptual categorisation deficit
A relatively mild form of apperceptive agnosia, usually caused by lesions to the right parietal lobe.
Patients can recognise prototypical views of objects, but not “unusual” views.
Cannot match different views of an object, i.e. fail to categorise percepts as belonging to the same object.
Could be a spatial problem - inability to decide which view one is looking at
What are the four types of associative agnosias?
- Visual Object Agnosia; inability to recognise objects
- Prosopagnosia; inability to recognise faces
- Alexia; inability to recognise words
- Topographical agnosia; inability to recognise familiar environments and landmarks
Describe the features of associative agnosia
Failures of recognition that cannot be attributed to faulty perception
Patients can copy drawings, discriminate shapes, segment images.
But cannot identify objects
Due to disconnection between intact perceptual input and memory? (peripheral)
Or loss of stores object representations? (central)
What types of lesions cause specific associative agnosias?
Lesions causing associative agnosias tend to be in the occipital and temporal regions (vOT?) – generally bilateral (can get unilateral lesions)
Objects (and especially words) more in the left hemisphere
Faces more in the right hemisphere
